Phase-sensitive frequency-modulation noise spectroscopy with a diode laser

We have demonstrated phase-sensitive FM noise spectroscopy, using the frequency noise of a diode laser. We obtained the reference signal for this experiment by sending part of the diode-laser beam through a Mach-Zehnder interferometer, which acted as a frequency discriminator. Both the reference beam (prepared with an interferometer) and the probe beam (which probed a Fabry-Perot cavity) carried amplitude noise converted from the intrinsic frequency noise. Mixing the two noise signals allowed us to measure the absorptive and dispersive responses of the sample. This method may provide a simple and sensitive spectroscopic tool.